This invention relates to semiconductor devices comprised of amorphous superlattice material. In particular, this invention relates to solar cells comprised of amorphous superlattice material.
Solar cells which are comprised of one layer of active material having a single optical bandgap are sensitive only to a limited range of photon energies. Photons with energies below the optical bandgap energy are not absorbed at all. Moreover the energy of photons in excess of the optical bandgap energy is dissipated as heat in the solar cell. In order to increase the efficiency of energy conversion of solar cells, it is desirable to match the energy gap of the material to the solar spectrum. Therefore, being able to choose the energy gap of the material for a solar cell appropriate will maximize the efficiency of the cell.
Another way to increase the efficiency is to cascade active material into a multijunction solar cell so that each active layer is responsive to a different region of the solar spectrum; see e.g. U.S. Pat. No. 4,017,332, U.S. Pat. No. 4,179,702, and U.S. Pat. No. 4,255,211.
A major problem for multijunction (tandem) solar cells is that for best performance the optical bandgaps of the individual active materials must fall within fairly narrow limits set by the solar spectrum. Thus materials that otherwise would be desirable from the point of view of ease of thin film formation or electrical properties for example, may not be suitable in tandem solar cell applications because of their optical gap. One example is hydrogenated amorphous silicon (a-Si:H). This material, when prepared under conditions that give optimal electrial properties, has a bandgap (1.7-1.8 ev) that is too low for the top layer in a two junction cell (1.8-1.9 ev is optimal) and too high for the bottom layer in a two junction cell (1.2-1.3 ev optimal).
This problem could be solved in principle with different semiconductor materials, such as the silicon rich alloys a-Si.sub.1-x Ge.sub.x :H and a-Si.sub.1-x C.sub.x :H; however, the best electrical performance in these materials is usually obtained with the pure Si(x=0) composition.
The present invention is a semiconductor device which includes amorphous superlattice materials in which the individual sublayers are prepared under conditions that give the best electronic properties; and the bandgap is controlled by varying the thickness of the individual sublayers.